1. Field of the Invention
The field of the invention is industrial controllers and more specifically controllers for injection molding machines.
2. Background Art
In the injection molding process, a plasticized material held in a "barrel" is forced under pressure, typically by means of a ram fitting within the barrel, through a nozzle in one end of the barrel. The plasticized material enters into a mold cavity under pressure where it solidifies into a molded part in conformance with the dimensions of the mold cavity. The part is then ejected from the mold and the process is repeated.
This injection molding process may be broken into four stages: plastication, injection, packing and holding.
In the plastication stage, solid pellets of the molding material are fed into the barrel where they are melted and forced to the front of the barrel by rotation of a screw forming part of the ram. As the molding material is melted by the mechanical action of the screw, the barrel begins to fill moving the screw and ram back from the nozzle. Control of the ram back-pressure may be used to ensure the melted molding material is at proper temperature and free from voids or air pockets.
In the injection stage, the rotation of the screw ceases and the ram is moved toward the nozzle to force the molding material through the nozzle into the mold cavity. The characteristics of the molding material or of the mold may require that certain parts of the mold cavity be filled at different rates. This may be accomplished by varying the speed or pressure of the ram during the injection stage.
In the packing stage, additional molding material is forced into the mold cavity to accommodate shrinkage of the molding material as it cools in the mold cavity.
In the holding stage, pressure is maintained on the molding material to control its density and/or flexibility. At the conclusion of the holding stage, the molded part shrinks away from the mold cavity prior to ejection of the part. Control of the ram pressure during the holding stage also may prevent distortions of or depressions in the part as it cools.
The ability to accurately vary the ram pressure or speed during the various stages of the injection molding cycle may be accomplished by means of profiles comprised of sequential segments having different programmed setpoints. A given profile may have multiple segments and setpoints to allow the programming of complex molding pressure functions during a given stage.
The use of multiple segments in a profile requires that the control system of the injection molding machine respond rapidly to the setpoint in each segment. High control speed may be obtained by operating the injection molding machine in an open-loop configuration. In an open-loop configuration, the setpoint is converted into a controlled variable value for the valve, i.e, a percent opening, to produce the desired ram speed or pressure. The conversion of the setpoint to a controlled variable value for the valve is based on empirical measurements of the valve's performance.
Although open-loop control provides rapid control of the injection molding machine, the accuracy of the control is low. The reason for this is that transfer function relating ram speed or pressure to valve opening is complex and subject to variations caused by numerous factors including the molding temperature, the mold type, and the composition of the plastic molding material.
One method of providing rapid and accurate ram control is to combine open-loop control with closed-loop control. As taught by co-pending application entitled: Injection Molding Controller with Controlled Variable Learning, Ser. No. 07/538,813 filed on June 15, 1990 and assigned to the same assignee as the present invention, the accuracy of rapid open-loop control may be refined, between cycles of the injection molding machine, by examining the controlled variable (i.e. the ram valve signal) during closed-loop control at the end of each segment and comparing the closed-loop controlled variable to the open-loop controlled variable. Differences are used to modify the setpoint for the next cycle to further improve the accuracy of the open-loop control.
This procedure may be inadequate for short stages of the injection molding cycle, particularly the injection stage which may comprise only a few percent of a total cycle time on the order of tens of seconds. In the injections stage and particularly for velocity control, the open-loop period of the control may be so long as to preclude large reductions of the control error by the closed-loop portion of the control. The open-loop controlled variable may therefore be very close in value to the closed-loop controlled variable even though the process variable (i.e. ram speed or pressure) is far from the setpoint value.